Icon colors are difficult to see
If you are talking about these icons:



You need to change the Themes>User Interface>Tab Inner, Selected and Outline colors.

Edit: OK, you mean these:



Unfortunately, those are built from source. See this link if you are interested in trying to modify them.

You can change the background and border though by going to User Preferences>Themes>User Interface>Tool and changing the Outline and Inner Options.